The invention relates to bit synchronization detection, and more specifically, to methods and systems for detection of bit synchronization boundary in Global Positioning System (GPS) signals.
GPS has provided many useful civilian applications such as automatic position reporting during emergencies, low-visibility harbor operations, navigation systems for drivers, hikers, and campers. GPS is a collection of earth-orbiting satellites, each satellite transmits a separate signal carrying information that allows GPS receivers to obtain good estimates of their position in real-time by locking onto at least three satellites. FIG. 1 illustrates transmission of an exemplary data bit according to the GPS standard. The GPS signal emitted by each satellite is modulated according to a unique Pseudo-Random Noise (PRN) code. A complete PRN code is composed of 1023 chips (bits), and the GPS signal is modulated with the PRN code that is repeated every millisecond (ms) as represented by label “1A” in FIG. 1. The receiver detects the GPS signal of a particular satellite by achieving a high correlation between the received signal and a shifted PRN code corresponding to the satellite. The receiver then uses the shifted PRN code to achieve synchronization with subsequent transmissions from the satellite.
GPS data bits are not protected by ordinary error correction algorithms such as inserting redundant bits, instead, each data bit is repeated twenty times for transmission. The period of the PRN code is 1 ms, so the period for transmitting one data bit is 20 ms after PRN code modulation. Label “1B” in FIG. 1 represents a time scale indicating epochs corresponding to the starting point of each 1023-chip represented by label “1A”. Label “1C” represents a data bit that will be transmitted utilizing twenty PRN code periods. The actual transmission bit rate for GPS is therefore 50 bps. When a receiver detects a GPS signal, it attempts to synchronize the data bit in the signal by determining and aligning the bit boundaries. Bit boundary determination during signal acquisition determines the start of each 20 ms data bit period, and can improve receiver sensitivity to weak signals. The bit boundaries are known only to within some multiple of 1 ms PRN code periods.
An epoch counter that repeatedly counts from one to twenty (or zero to nineteen as represented by label “1B” in FIG. 1) without alignment is introduced in a conventional histogram approach for bit boundary determination. This histogram approach breaks each 20 ms data bit period into twenty 1 ms epoch periods, and senses sign changes or data bit transition between successive epochs. A corresponding counter out of twenty counters is incremented for each data bit transition sensed. After an appropriate interval, the bit boundary can be determined through voting between the twenty counters. Obtaining an adequate result with the histogram approach is, however, time-consuming. A time-consuming data bit demodulation will significantly increase TTFF (time to first fix), which is the most important performance evaluation parameter of a GPS receiver.